Photovoltaic cell array

ABSTRACT

In a photovoltaic cell which comprises: a substrate, a bottom electrode, a first layer of cadmium sulfide, a second layer of cuprous sulfide forming a barrier junction with said first layer and a top electrode, the improvement wherein said substrate is an insulative ceramic material and the bottom electrode is a conductive ceramic layer fused to said substrate. Said conductive layer is optionally coated with a metal having a high electrical conductivity.

This application is a continuation-in-part of copending application Ser.No. 747,849, filed Dec. 6, 1976 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to solar cell arrays and a method ofmaking them. More particularly, the invention relates to an integratedarray of thin-film, photovoltaic cells connected in series and/orparallel and a method of making it.

2. The Prior Art

Since each commonly known, individual solar cell generates only a smallamount of power, usually much less power than is required for mostapplications, the desired voltage and current is realized byinterconnecting a plurality of solar cells in a series and parallelmatrix. This matrix is generally referred to as a solar cell array, andgenerates electrical energy from solar radiation for a variety of uses.

Solar cell arrays can be made manually by bonding the individual cellsto a suitable support in the desired configuration and connecting, e.g.,by soldering, the electrical leads of the individual cells in thenecessary manner to give the desired voltage and current. Manualconstruction of arrays suffers from a number of disadvantages, includingcumbersome and difficult construction methods, expense, faultyconnections, and the like. An integrated array and method of making itis described in U.S. Pat. No. 3,483,038, issued Dec. 9, 1969 to Hui etal. The patented array comprises a substrate of flexible plasticinsulating material such as polyimide plastic to which a plurality ofserially connected individual cells are integrally united. Theindividual cells comprise a bottom electrode of a three-layered metalfilm covered by a film of an n-type semiconductor such as cadmiumsulfide and a film of p-type semiconductor such as cuprous sulfide toform a barrier layer, and a top electrode of a thin film of metal suchas tellurium.

The use of flexible plastic films of the prior art as substrates hascertain disadvantages. Many of the plastics are air and water permeableto a certain degree which makes it impossible to completely hermeticallyseal the final cell array and can result in degradation of the cellcomponents over a period of time as air and water diffuse into the cell.It is frequently difficult to bond the metal electrode to the plastic.The plastic substrate is too flexible for many uses and cellmanufacturing techniques, so rigidity must be built in by laterencapsulation, or the flexible substrate must be supported with a rigidstructure. Bonding the plastic film to a rigid structure can producedegradation of the film by mechanical deforming of the film from bondingpressures, or from heat used in thermal bonding or from solvents used inadhesives. Many plastics have absorption peaks in the infared, resultingin high temperatures in use when these plastics are used in photovoltaiccell construction. Flexing of the substrate over a period of time of usetends to break down the films in the cells, reducing their output andshortening their life. The inherent flexibility of plastic substratematerial increases processing complexity, since in order to obtain highresolution of array cells, the plastic film can not be subjected tostresses during the manufacturing process which would cause stretchingand buckling. Use of a plastic substrate limits the temperature to whichthe cell can be subjected during fabrication.

Abrahamsohn in U.S. Pat. No. 3,376,163, issued Apr. 2, 1968, illustratesthe use of commercially available conductive glass as a cell substrate.These are glasses that have a thin layer of conductive tin oxide on thesurface. These glasses are typically produced by spraying a solution oftin chloride on hot glass, above 800° F. See U.S. Pat. No. 2,648,753,issued Aug. 11, 1953, to Lytle for a description of the process. Thisprocess has several disadvantages. Because the tin salt solution issprayed onto a hot glass surface, the conductive layer is continuous onthe surface of the glass and it is not possible to obtain thecomplicated grid pattern needed to obtain a parallel or series array ofcells on a simple substrate.

SUMMARY OF THE INVENTION

The integrated array of the present invention comprises a plurality ofthin-film photovoltaic cells connected in series and/or parallel andintegrally united to a substrate of insulating ceramic. Each of thecells comprises a bottom electrode of conducting ceramic bonded to thesubstrate, a film of a first semiconductor material of one typeconductivity covering the bottom electrode except for a minor portionwhich serves as a lead electrically connected with an electrode of anadjacent cell, a film of a second semiconductor material of an oppositetype conductivity forming a p-n junction with the first semiconductormaterial, and a top electrode extending to and connecting with anelectrode of an adjacent cell. One of the electrodes in each end cell inthe array serves as a terminal for the array. A thin metallic film canbe deposited on the bottom electrode to produce good adhesion and goodohmic contact between the bottom electrode and the first semiconductorfilm.

The insulative ceramic substrate and conductive ceramic electrodeprovide a number of advantages over the prior art. Good bonds betweenthe substrate and bottom electrode and good adhesion of the cadmiumsulfide film to the bottom electrode are obtained. The ceramic substrateprovides sufficient rigidity to reduce flexurally induced damage tofinished arrays. Higher temperatures can be used in the manufacture ofthe arrays. The ceramic substrate and conductive ceramic electrode haveexcellent vacuum processing characteristics, allowing high temperaturesof operation with minimal outgassing. The rigidity of the substrate ofthis invention allows a high degree of resolution of array cells to beobtained during manufacture. Good hermetic seals can be obtained toexclude air and water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of one embodiment of an integrated array withcells in a side by side relationship and connected serially.

FIG. 2 is a cross-sectional view of cell 13 taken along line 2--2 ofFIG. 1 showing the layers of the individual cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2 of the drawings, there is shown an integratedarray of a plurality of serially connected, thin-film photovoltaic cells11, 12 and 13, integrally formed on and united to a substrate 10 ofinsulating ceramic material. Although the integrated array described andillustrated herein has only three cells for the sake of clarity ofexplanation, the integrated array may have as many cells as areconvenient and practical for any particular application. A row of cellsin a side by side configuration and connected in series is shown;however, a plurality of rows of cells can be provided wherein some orall of the rows are connected in parallel with each other, as desired,to provide a predetermined power output.

The substrate is an insulative ceramic and is capable of withstandinghigh temperatures. The ceramic substrate has sufficient rigidity tostructurally support a number of individual cells. While glass or otherceramic materials can be used, a particularly suitable substrate issheet metal coated, preferably on one side, with a thin layer ofceramic. The metal imparts structural strength to an array and can bemade sufficiently thin to impart some degree of flexibility if desired.Moreover, the use of metal facilitates overall fabrication of solarcollectors, especially where heat exchange means are provided to removethermal energy. Ceramic coated metal is readily available as it is usedwithin the appliance industry for refrigerators, stoves, and the likeand in the construction industry as panels for commercial buildings suchas service stations. A suitable material is, for example, "Mirawal," atrademark product of Kaiser Aluminum Company for cold rolled low-carbonsteel sheets having a continuous coating of vitreous enamel on at leastone side of the sheet. Desirable thickness of the sheet metal rangesfrom about 0.005 to about 0.030 inches, although thicknesses from about0.012 to about 0.025 inches are also suitable. The thickness of theceramic coat should be sufficient to insulate the sheet metal from thebottom electrode and desirably ranges from about 0.0015 to about 0.012inches, although thicknesses from about 0.0025 to about 0.0065 are alsosuitable. The ceramic coating should be free from pinholes or otherdefects which would tend to result in short circuits to the sheet metal.

The individual cells are integrally formed on and united to thesubstrate. Any desired design configuration can be used other than thatgiven in FIG. 1 for illustrative purposes. Each cell comprises a numberof thin layers which, for the sake of clarity, are shown in anexaggerated manner in the drawing. The construction of a typical cell,e.g., cell 12, will be described in detail.

Substrate 10 comprising mild steel 16 coated with a thin film ofinsulating vitreous enamel 17 is coated with a thin layer of conductiveceramic material 18. This conductive ceramic layer 18 provides thebottom electrode of the cell assembly and is represented by the areaABCDEF in FIG. 1 for cell 11. The conductive ceramic layer is applied asa paste to substrate 10 by suitable means such as silk screening, handor machine dipping or by spraying through a suitably apertured mask.After application to substrate 10, ceramic conductive layer 18 issuitably cured. Such curing process typically involves first drying attemperatures of from about 75° to about 150° C for times ranging fromabout 0.05 to about 1 hour, preferably from about 0.1 to about 1 hour.Higher temperatures will require short drying times and vice-versa.Temperatures outside the above limits are useful but too low atemperature will require long drying time which will be uneconomical,whereas too high a drying temperature can result in poor adherence ofand/or voids in the conductive ceramic layer due to rapid evaporation ofsolvent from the paste. Drying is followed by burn-out in air of organicmaterials such as organic screening materials such as resins,plasticisers, hardeners, wetting agents, thickeners and the like used inthe paste formulations. Burn-out is typically carried out in air forfrom about 0.05 to about 1 hour and at temperatures ranging from about100° to about 500° C, preferably from about 150° to about 500° C andmore preferably from about 250° to about 400° C. After burn-out, theconductive ceramic paste is fired in a neutral atmosphere such asnitrogen, argon, helium, and the like for about 0.05 to about 1 hour atpeak temperatures ranging from about 500° to about 1200° C, preferablyfrom about 500° to about 1100° C, more preferably from about 500° toabout 1000° C and yet more preferably from about 500° to about 950° C.This firing results in the sintering of the paste into a solid layer,and the adherence of the layer to the ceramic surface 17 of thesubstrate. Upon sintering the edges of the bottom electrode will befound to be rounded rather than sharp, which facilitates vapordeposition of the cadmium sulfide film 20 over the edges of the bottomelectrode to the substrate 10, thus protecting the bottom electrode fromdeposition of copper during the barriering process. A copper layer wouldestablish electrical contact between the bottom electrode and thebarrier layer. Also, if sharp corners were present instead of roundedcorners, the possibility of thin spots or pin holes in the cadmiumsulfide film exists.

Suitable conductive ceramic pastes comprise: from about 30 to about 85percent by weight and preferably from about 40 to about 75 percent byweight of a metal having a high electrical conductivity, such as, forexample, nickel, silver, gold, palladium, platinum and copper withcopper being preferred; from about 1 to about 40, preferably from about1 to about 20 percent by weight of a metal oxide capable of forming,upon firing, a metal oxide - silica complex, thus allowing theconductive metal to wet the complex of the fired paste, copper oxidebeing preferred and of the copper oxides, cuprous and cupric, cuprousoxide being preferred; from about 3 to about 30 percent by weight,preferably from about 3 to about 20 percent by weight of a ground (< 400mesh) glass frit (powder), optionally from about 1 to about 20 percentby weight of a fluxing agent such as bismuth trioxide, antimonytrioxide, lead oxide and the like, bismuth trioxide being preferred;from about 1 to about 10 percent by weight of a suitable screening agentsuch as ethyl cellulose, nitrocellulose and the like; and from about 1to about 50, preferably from about 5 to about 45 percent by weight of asuitable solvent such as turpentine, pine oil, naptha, and the like. Thescreening agent is typically used in solution with a suitable solventsuch as turpentine, pine oil, naptha, and the like. The concentration ofthe screening agent in the solvent typically ranges from about 10 toabout 50 percent by weight. A paste composition that when dried andfired has been found to produce a copper-containing conductor with goodelectrical characteristics, excellent soldering and electroplatingcharacteristics and good adhesion to the substrate is as follows: 66%wt. copper powder sieved to <400 mesh (37 microns), (Fernlock D-100,U.S. Bronze, Flemington, N.J.), 9% wt. cuprous oxide, 5% wt. bismuthtrioxide, 5% wt. glass frit (Pemco S-2120-P manufactured by SCM Corp. ofBaltimore, Md.), 12% wt. of MM5 ethyl cellulose based screening vehicle(1.2% ethyl cellulose; 0.6% Poly-Pale resin hardener, Hercules; 0.12%CO-430 non-ionic wetting agent, GAF; 10.08% pine oil) and 3% wt. pineoil (solvent). The fired conductive ceramic layer typically has athickness ranging from about 0.3 mils to about 2.0 mils, preferably fromabout 0.7 mils to about 1.2 mils. After sintering of this type of paste,the electrically conductive metal-containing bottom electrode willcontain from about 70 to about 85 percent by weight of a metal in thezero valent state, from about 10 to about 20 percent by weight ofceramic and from about 5 to about 10 percent by weight of the metaloxide which is believed to form a complex with said ceramic.

Ceramic pastes having lower metal contents than the above describedpastes can also be used. With lower metal content, the sinteredconductive ceramic layer is advantageously electroplated with a metalhaving a high electrical conductivity such as, for example, silver,gold, platinum or copper, with copper being the preferred metals. Theelectrodeposition processes used are those which are well known in theart and are suitably adapted by those skilled in the art to theparticular metal being deposited. Suitable conductive ceramic pastesencompassing the lower metal contents comprise from about 5 to about 40and preferably from about 10 to about 30 percent by weight of a metalhaving a high electrical conductivity; such as, for example, nickel,silver, gold, palladium, platinum and copper with copper beingpreferred; from about 15 to about 75, preferably from about 20 to about40 percent by weight of a metal oxide capable of forming, upon firing, ametal oxide-silica complex, thus allowing the conductive metal to wetthe complex of the fired paste, copper oxide being preferred; and of thecopper oxides, cuprous and cupric, cuprous being preferred; from about 2to about 60, preferably from about 3 to about 50 percent by weight of aground glass frit (<400 mesh); optionally from about 1 to about 20percent by weight of a fluxing agent such as bismuth trioxide, antimonytrioxide, lead oxide and the like, bismuth trioxide being preferred;from about 0.05 to about 10 percent by weight of a suitable screeningagent such as ethyl cellulose, nitrocellulose and the like; and fromabout 1 to about 50, preferably from about 5 to about 45 percent byweight of a suitable solvent such as turpentine, pine oil, naptha, andthe like. The screening agent is typically used in solution with asuitable solvent such as turpentine, pine oil, naptha, and the like. Theconcentration of the screening agent in the solvent typically rangesfrom about 2 to about 40 percent by weight. A paste composition thatwhen dried and fired has been found to produce a low metal-containingconductive ceramic film that is readily electroplatable and has goodadhesion to the substrate is as follows: 15% wt copper flake powder withthe individual flakes having, in the average, a length of about 5-15microns and a width of about 1-5 microns (MD 955 copper Flake-AlcanMetals), 28% wt. cuprous oxide, 12.4% wt. bismuth trioxide, 12.4% glassfrit (S-2120-P Glass, 400 mesh (37 microns) PEMCO), 28.2% MM22ethylcellulose based screening vehicle (2.82% ethyl cellulose; 1.41%Poly-Pale resin hardener, Hercules; 0.28% CO-430 non-ionic wettingagent, GAF; 23.69% pine oil), 4% wt. pine oil. After sintering of thistype of paste, the electrically conductive, metal-containing bottomelectrode will contain from about 15 to about 70 percent by weight of ametal in the zero valent state, from about 5 to about 50 percent byweight of ceramic and from about 5 to about 60 percent by weight of themetal oxide.

In general, the conductive ceramic paste will comprise from about 5 toabout 85 percent by weight and preferably from about 10 to about 75percent by weight of the metal having a high electrical conductivity,such as, for example, nickel, silver, gold, palladium, platinum andcopper with copper being preferred; from about 1 to about 75, preferablyfrom about 1 to about 60 percent by weight of a metal oxide capable offorming, upon firing, a metal oxide - silica complex, thus allowing theconductive metal to wet the complex of the fired paste, copper oxidebeing preferred, and of the copper oxides, cuprous and cupric, cuprousoxide being preferred; from about 2 to about 60, preferably from about 3to about 50 percent by weight of a ground glass frit (<400 mesh);optionally from about 1 to about 20 percent by weight of a fluxing agentsuch as bismuth trioxide, antimony trioxide, lead oxide and the like,bismuth trioxide being preferred; from about 0.05 to about 10 percent byweight of a suitable screening agent such as ethyl cellulose,nitrocellulose and the like; and from about 1 to about 50, preferablynitrocellulose and the like; and from about 1 to about 50, preferablyfrom about 5 to about 45 percent by weight of a suitable solvent such asturpentine, pine oil, naptha, and the like. The screening agent istypically used in solution with a suitable solvent such as turpentine,pine oil, naptha, and the like. The concentration of the screening agentin the solvent typically ranges from about 2 to about 40 percent byweight. After sintering, the electrically conductive, metal-containingbottom electrode will contain from about 15 to about 85 percent byweight of zero valent metal, from about 5 to about 50 percent by weightof ceramic and from about 5 to about 60 percent by weight of the metaloxide which is believed to form a complex with said ceramic.

Other methods of producing metal-containing conductive ceramic films areknown in the art and are considered within the scope of this invention.

When the conductive metal in the conductive ceramic layer is a metal ofthe type which would form a rectifying barrier with cadmium sulfide orwould oxidize on the surface to form a poor ohmic contact with cadmiumsulfide, then it is desirable to coat the conductive ceramic with atransition layer 19 of metal or metal alloy which does not have theabove drawbacks. Copper, for example, would form a barrier with cadmiumsulfide. Aluminum would oxidize and the layer of exposed aluminum oxidewould be an insulator and would give a high resistance contact withcadmium sulfide. Suitable metals for the metal transition layer includegold, silver, platinum, cadmium, zinc and alloys thereof. Zinc ispreferred. More than one layer of metal may be deposited on theconductive ceramic substrate. A preferred embodiment comprises thebottom electrode having deposited thereon a layer of copper and havingfurther deposited thereon a layer of zinc.

The metal transition layer is applied to the conductive ceramic layer inany suitable manner. For example, it may be applied by vapor depositionthrough a suitably apertured mask, by electrodeposition from a solutionof salts of the metal, or by contact with a molten bath of metal. Theseprocesses are well known in the art and will be suitably adapted by oneskilled in the art to the particular metal being deposited. The metaltransition layer will be applied in an amount, for example, ranging fromabout 0.0001 to about 0.01 gm/sq cm. preferably from about 0.001 toabout 0.002 gm/sq cm, with zinc being a preferred metal.

The bottom electrode of the cell comprises either the conductive ceramiclayer, or the conductive ceramic layer covered with a metal transitionlayer when used. Upon this bottom electrode a semiconductor material ofn-type conductivity such as cadmium sulfide film 20 is deposited. Thiscan be done in a known manner, such as through a suitably apertured maskfrom the vapor state, in an amount of between about 0.05 gm/sq cm andabout 0.005 gm/sq cm. The cadmium sulfide film 20 covers and completelyoverlaps all but a small portion of the bottom electrode, this areabeing represented by ADEF shown in FIG. 1 for cell 11. The uncoveredportion is represented by the area ABCD and can be used subsequentlyeither for electrical connecting means to an adjacent cell, such as thetop electrode of an adjacent cell to make a series connection therewithas shown in FIG. 1, or for a negative output terminal such as 14. It isimportant that the cadmium sulfide film 20 in each of the cells 11-13,for example, overlaps the periphery of the bottom electrode 19, such as,the edges AF, DE, and FE thereof, and extends to the surface ofsubstrate 10 because the subsequent overlapping films and the topelectrode in each cell must not contact the bottom electrode layers 19or 18 thereof. As discussed before, the use of a sintered ceramic bottomelectrode facilitates this covering, since the sintering processproduces rounded and filleted edge surfaces which are easily covered bycadmium sulfide during the vapor deposition process.

After deposition of the cadmium sulfide, a strip of insulating material24, such as silicon dioxide or cured epoxy resin is deposited along theedge AD of the cadmium sulfide layer 20 as well as upon most of thesubstrate area extending beyond edge AD. The purpose of the insulatingfilm is to prevent the cuprous sulfide film 21 from coming in contactwith the bottom electrode 19 which would short out the p-n junctionbetween the cadmium sulfide and cuprous sulfide layers. This insulatinglayer 24 also allows the top electrode from one cell to be connected tothe exposed part of the bottom electrode of an adjacent cell withoutshorting to its own bottom electrode.

The surface of the cadmium sulfide film 20 may be etched withhydrochloric acid for about 4-5 seconds, if desired, before the cuproussulfide films are formed thereon, as described in Tanos, U.S. Pat. No.3,480,473. The cuprous sulfide film 21 is formed in a suitable fashionsuch as, for example, deposition from the vapor state through a suitablyapertured mask, over the cadmium sulfide film 20, or by contacting thecadmium sulfide film 20 with an aqueous solution of a cuprous salt as,for example, a cuprous chloride or bromide solution, as described inKeramidas, U.S. Pat. No. 3,374,108. The cuprous sulfide film 21 willhave a thickness between about 1000A and about 10,000A.

A top electrode is fixed to each cell. The top electrode can suitably beany material of high electrical conductivity. It must allow, or beshaped to allow, light to reach the cuprous sulfide layer. Suchelectrodes are known in the art. Preferably, and as shown in FIG. 1, thetop electrode comprises a plurality of electrode strips 22 whichterminate in tab 23 at one end, AD, and in a bar at end FE. Theelectrode strips are placed in electrical contact with the cuproussulfide film 21 while tab 23 extends over insulating strip 24 toelectrically contact a portion, BCJI, of bottom electrode 19 of anadjacent cell to electrically connect the two cells in series. As shown,the top electrode for cell 12 connects with the bottom electrode of cell11, and the bottom electrode of cell 11, and the bottom electrode ofcell 12 is connected to the top electrode of cell 13. For end cell 11,the top electrode tab will be the positive terminal 15 for the array.

The top electrode may be provided in any known manner, such as bydeposition through a suitably apertured mask from the vapor state overthe cuprous sulfide film 21. Alternatively, the top electrode may bevapor deposited on a flexible insulating film such as Mylar, Aclar, orTFE and then the film 25 pressed onto the cell with top electrode strips22 in contact with cuprous sulfide film 21 and held in place with lighttransmissive epoxy cement 26. The top electrode can also be a grid, ormesh, of fine metal wire which is attached to the cuprous sulfide film.The top electrode may be any suitable electrical conductor having a highelectrical conductivity, which forms a good ohmic contact with cuproussulfide film 21, and which will not form a p-n barrier junction with thecuprous sulfide film. Suitable electrical conductors are, for example,metals such as gold, platinum and silver.

The finished cell assembly is typically heat treated, e.g., at 200° Cfor 10 minutes and sealed with a protective light transmitting coating,a film or plate 27 of a material such as glass or the like. The filmshould be impervious to oxygen and water vapor which would degrade thecell.

In operation, the cells, 11, 12, and 13 convert light into electricalenergy when they are exposed to light. In each cell, light energy passesthrough the area not covered by top electrode strips 22 to the cuproussulfide film 21 where it is at least partially absorbed therebyproducing a voltage between the bottom electrode and the top electrode.Since this voltage for a photovoltaic, cadmium sulfide cell is typicallyabout 0.4-0.5 volts, the cells 11-13 are connected in series to providea desired voltage. The current capacities at the desired voltage may beincreased by connecting a plurality of the serially connected rows ofcells in parallel.

In generic terms the invention provides an integrated array ofconnected, thin-film photovoltaic cells comprising:

a substrate of electrically insulating ceramic material;

a plurality of said cells integrally united to a major surface of saidsubstrate in a spaced-apart relationship, each of said cells comprising:

an electrically conductive, metal-containing, ceramic bottom electrodecoated on said substrate and bonded thereto by fusing or sintering;

a film of first semiconductor material of one type conductivity coveringand overlapping all but a portion adjacent an edge of said bottomelectrode;

a film of second semiconductor material of an opposite type conductivityon said first film of semiconductor material and forming a p-n junctiontherewith; and,

a top electrode, capable of transmitting radiant energy, in contact withsaid second semiconductor material;

said top electrode of one cell being connected to selected top andbottom electrode of an adjacent cell to provide electrically series orparallel arrangement of the cells.

This invention further provides a method of making an integrated arrayof connected, thin-film photovoltaic cells comprising:

providing a substrate of insulating ceramic;

providing on a major surface of said substrate a plurality ofelectrically conductive, ceramic bottom electrodes, a separate one ofsaid bottom electrodes being for each of said cells, by coating thesubstrate with a paste of conducting ceramic, and heat treating to dryand bond the conductive ceramic to the substrate;

coating each of said bottom electrodes, except for a portion adjacent anedge thereof, with a film of first semiconductor material of one typeconductivity;

coating each of said films of one type conductivity with a relativelythin film of second semiconductor material of an opposite typeconductivity, and forming a p-n junction therewith;

attaching a top electrode on each of said second films, selected ones ofsaid top and bottom electrodes of one cell extending to, and makingconnection with, selected top and bottom electrodes of adjacent cells toconnect the cells in series or in parallel arrangement.

What is claimed is:
 1. In an integrated array of connected, thin film,photovoltaic cells comprising:a substrate of electrically insulativematerial; a plurality of thin film photovoltaic cells integrally unitedto a major surface of said substrate in a spaced-apart relationship,each of said cells comprising: an electrically conductive bottomelectrode united to said major surface of said substrate; a film offirst semiconductor material of one type conductivity covering andoverlapping all but a portion adjacent an edge of said bottom electrode;a film of second semiconductor material of opposite type conductivityand forming a p-n junction with the first semiconductor material and; atop electrode, in contact with said second semiconductor material andwhich allows radiant energy to pass into the second semiconductormaterial, selected ones of said top and bottom electrodes of one cellbeing connected to selected top and bottom electrodes of adjacent cellsto connect said cells in a series or parallel arrangement; theimprovement wherein said substrate comprises an insulative ceramic andsaid bottom electrode comprises an electrically conductive ceramicunited to said major surface of said substrate.
 2. The integrated arrayof claim 1 wherein said substrate comprises sheet metal coated at leaston one side with a thin layer of insulating ceramic.
 3. The integratedarray of claim 1 wherein the bottom electrode comprises from about 15 toabout 85 percent by weight of a metal, from about 5 to about 50 percentby weight of ceramic material and from about 5 to about 60 percent byweight of a metal oxide.
 4. The integrated array of claim 3 wherein thebottom electrode comprises from about 70 to about 85 percent by weightof the metal, from about 10 to about 20 percent by weight of the ceramicmaterial and from about 5 to about 10 percent by weight of the metaloxide.
 5. The integrated array of claim 3 wherein the bottom electrodecomprises from about 15 to about 70 percent by weight of the metal. 6.The integrated array of claim 5, wherein said bottom electrode comprisescopper and cuprous oxide, having deposited thereon a layer of copper andhaving further deposited thereon a layer of zinc, said firstsemiconductor material is cadmium sulfide and said second semiconductormaterial is cuprous sulfide.
 7. The integrated array of claim 6, whereinthe substrate comprises sheet metal coated on at least one side with athin layer of insulating ceramic.
 8. The integrated array of claim 3,wherein said conductive ceramic is coated with at least one metal layer,one of which makes an ohmic contact with the first semiconductormaterial.
 9. The integrated array of claim 6, wherein said bottomelectrode comprises copper and cuprous oxide, said metal coating iszinc, said first semiconductor material is cadmium sulfide and saidsecond semiconductor material is cuprous sulfide.
 10. The integratedarray of claim 7, wherein the substrate comprises sheet metal coated onat least one side with a thin layer of insulating ceramic.